Why are you asking for a 16 bit depth buffer in OpenVG? OpenVG is 2D only so a depth buffer makes no sense, take that out and you should be allowed the alpha mask. eglChooseConfig() failed when you asked for both a depth buffer and an alpha mask buffer because there is no config that has both a depth buffer and an alpha mask buffer. If you had queried the number of configs available matching your attributes you would have seen that there were none.

Not overly I wouldn't think. I've only done OpenVG drawing into a VGImage but no tests as to the speed of it (a static one-time draw into the VGImage right at the start and then using the image multiple times).